This invention was made with Government support under F30602-03-C-0119 awarded by the AFRL. The Government has certain rights in this invention.
All patents, patent applications, and publications cited within this application are incorporated herein by reference to the same extent as if each individual patent, patent application or publication was specifically and individually incorporated by reference.
The invention relates generally to optical modulators, and more specifically to analog optical modulators. Optical modulators encode radio frequency (RF) onto an optical carrier. When an RF signal is encoded onto an optical carrier, there is a fundamental harmonic RF signal accompanied by higher order RF harmonic signals, all of which are encoded onto the optical carrier. The presence of higher order harmonic signals creates distortion of the fundamental harmonic RF signal, which decreases signal fidelity. High fidelity encoding of the RF signal onto the optical carrier can occur if the transfer function of the modulator is linear. The quantitative measure of optical modulator linearity is the spur-free dynamic range (SFDR). The spur free dynamic range of a modulator is measured at a detector that converts the optical signal into an RF signal. There is always electronic noise from various sources in the detector, and the highest noise level is referred to as the noise floor. The intensity of the current generated at the detector is greatest for the fundamental harmonic of the RF signal. The SFDR is measured as the difference between the intensity of the fundamental harmonic signal and the intensity of the first higher order mode to appear above the intensity of the noise floor. The higher order harmonic is typically the third order harmonic.
Analog optical modulators ideally have a linear transfer function. Analog optical modulators may include optical devices such as, alone or in combination, Mach-Zehnder modulators or directional couplers, for example see W. B. Bridges IEEE Trans. Microwave Theo. Tech. 43(9), 2184 (1995). A directional coupler has a coupling region where two optical waveguides are coupled so that light propagating in one optical waveguide can be switched to the other optical waveguide. The distance required for light to switch from one optical waveguide to the other optical waveguide is known as the coupling length. The coupling length may be influenced by a number of factors including the separation between optical waveguides. Light switching within the coupling region may be controlled by changing the refractive index of at least one of the optical waveguides. Typically, the refractive index is changed by applying a voltage across the optical waveguide. The directional couplers can then be used as a modulator if the output of at least one of the waveguides is monitored (i.e., the optical signal in one waveguide disappears when the light is switched to the other waveguide by applying a modulating voltage).
Although many designs for analog optical modulators have been tested and some commercialized, there is still a need for higher linearity as measured by the spur-free dynamic range.